Is there a way to get the metadata only instead of pulling the data itself for tap-postgres. I want to know how many rows and columns exists in the source.

Yes,

meltano invoke --dump=catalog tap-postgres > catalog.json

the data will be in the file. Note that how many rows is dependent on the source. Also note that

invoke

has caching logic in place so that discovery doesn't run every time, so you may want to remove that cache

Thanks, but how do I clear the cache?